The longterm objective of this proposal is to characterize the interaction of the platelet activating factory (PAF) family of ligands with specific cell membrane receptors and to establish the pathways by which that initial interaction leads to PAF induced activities. A main hypothesis of this proposal is that the PAF family of ligands exerts its many actions through more than one membrane receptor subtype. The alternative is that several functions can be initiated subsequent to activating a single receptor type (via activation of alternate postreceptor pathways). Three approaches are used to address this issue. First, we will attempt to detect and clone more than one gene for PAF receptors present in the human genome. Initial identification will be achieved by a strategy of homology using both the polymerase chain reaction technique and screening a genomic library. Identified genes will be cloned and expressed by transfection into COS-7 cells. Receptor specificity will be demonstrated by radioligand binding. The second approach to subtypes will determine whether receptor subtypes are expressed on the membranes of normal cells. Radioligand binding techniques will be used to compare PAF receptors of single types or subtypes in transfected cells with those found on human polymorphonuclear leukocytes (PMNs). If these studies implicate receptor subtypes, but only one gene is identified, a functional expression strategy will be added, employing mRNA isolated from HL60 cells. Third, PAF-receptor- specific antibodies will be prepared and assessed for inhibition of PAF- induced activities in PMNs (PAF-induced priming and enzyme secretion). If the antibodies block on PAF function but not others, they will link receptor subtypes with particular cell functions. These studies will determine if PAF induces a variety of functions via different receptor subtypes or whether binding of different ligands to one receptor can alter subsequent pathway. The studies will also select among three possibilities for PAF antagonists: The existing PAF antagonists are selective inhibitors for certain functions, that further development of selective antagonists is needed or that postreceptor pathways should be pursued for selective modulation of PAF functions.